Explore the mechanical properties of nano-scaled materials
Prof. OSHIMA Yoshifumi, Japan Advanced Institute of Science & Technology (JAIST), Japan
14:00-16:00, October 16, 2023
Yiucheng Lecture Hall (500), Xu Zuyao Building
Dr. Yoshifumi Oshima is a professor of Japan Advanced Institute of Science and Technology (JAIST). From 2023, he becomes director of nanomaterials and devices research area in JAIST. He received his doctor degree from Tokyo Institute of Technology in 2006 in Material Science. His research interests are novel physical and chemical properties of atomic scaled materials. He developed a miniature scanning tunneling microscope (STM) which can be installed in ultra-high vacuum transmission electron microscopy (UHV-TEM) and proved that gold nanowires have quantized conductance and so on. Recently, he established microscopic nanomechanical measurement method to estimate very low surface Young’s modulus of gold nanocontacts and also estimate individual bond stiffness in a Pt atomic chain. He received the society award (Seto prize) of the Japanese Society of Microscopy in 2011.
As materials continue to be downsized, interest in the properties of nanomaterials is growing. To date, a vast amount of research has been conducted on the physical properties of nanomaterials, and it has been specifically shown that they differ from bulk crystals. However, when atoms become so small that they can be counted, the physical properties of nanomaterials become sensitive to the atomic arrangement, for example, the electrical conductance changes between 1 and 2 G0 (G0: quantized unit of conductance), corresponding to the atomic arrangement in two atomic strands. It is important to clarify the relationship between the atomic arrangement of nanomaterials and their physical properties. Therefore, our group has developed some kinds of in-situ TEM holder to clarify the relationship between atomic structure and its physical properties. In this presentation, I will mainly introduce studies of nanomachanics.
Recently, we have developed an in-situ transmission electron microscope (TEM) with a quartz length-extension resonator (LER) as the force sensor, which enabled us to measure the elastic property simultaneously with identifying the atomic configuration [1,2]. The quartz LER has an advantage of high Q-factor and stiffness so that the stiffness can be obtained with a sub N/m precision.
We investigated the elasticity of Au nanocontacts with an axis along the  direction (Au  NCs) in the thinning process by stretching. The Au  NCs had an hourglass-like neck shape. It became progressively thinner as a layer of (111) atoms with a smaller cross-sectional area was inserted with each 0.24 nm stretching. Based on this observation, the stiffness of the inserted (111) atomic layer was determined from the difference in stiffness before and after insertion, and Young's modulus of the inserted (111) layer was calculated considering the shape and size of this (111) atomic layer. We found that Young's modulus gradually decreased from about 80 GPa to 30 GPa as the size decreased below 2 nm. From this result, the Young's modulus of the outermost layer could be estimated to be about 22 GPa, which is about 1/4 of the bulk value (90 GPa). Our TEM observations of surface atom diffusion also supported such softening of the surface .
We also successfully measured the width dependence of Young's modulus of monolayer MoS2 nanoribbons. Young's modulus was found to increase with narrowing width, and we were able to reproduce this using first-principles calculations. This was explained by the increase in bond strength due to buckling of Mo atoms at the edge of the nanoribbon .
 J Zhang et al. Nanotechnology 31, 205706 (2020).
 J Zhang et al. Nano Letters 21, 3922–3928 (2021).
 J Zhang et al. Physical Review Letters 128, 146101 (2022).
 C. Liu et al. Advanced Science, 2023, 2303477 in accept.